1. Field of the Invention
The present invention relates to a corrosion resistant structural alloy for electrolytic reduction equipment for treatment of spent nuclear fuel, more particularly, to a corrosion resistant structural alloy for electrolytic reduction equipment used for treatment of spent nuclear fuel, wherein Cr, Si, Al, Nb and Ti are added to a nickel (Ni) based substrate to form an oxide coating film which is stable in a LiCl—Li2O molten salt, in addition, a process for formation of the same and use thereof.
2. Description of the Related Art
An electrolytic reduction process of an oxide based spent nuclear fuel generally includes introducing the oxide based spent nuclear fuel into an anode in a LiCl—Li2O molten salt, applying electricity to reduce Li2O, and then, using the reduced Li to reduce nuclear fuel components. Such a process is very severe upon most of structural metal materials in chemical aspects due to strong corrosive properties of Li2O and oxygen generated at a cathode. Especially, fuel components react with a structural material during a reduction process so as to form a liquid phase, thus accelerating corrosion. Accordingly, a reactor for electrolytic reduction and at least one structural material used therein must have durability in a LiCl molten salt atmosphere including oxygen, a transuranic (TRU) component and Li2O at 650□. However, commercially available alloys lack enough corrosion resistance to endure the above described condition and cannot ensure stability in operating for a long period of time. For requirement of high-temperature corrosion resistance, Ni-based alloys are mainly used. For an alloy requiring corrosion resistance in specific conditions, the alloy may have a constitutional composition varied according to uses thereof. U.S. Pat. No. 4,034,142 (Jul. 5, 1977), entitled “Superalloy base having a coating containing silicon for corrosion/oxidation protection,” describes an alloy which was developed for gas turbine engines, which has a large content of Co and was also used as a coating material. Accordingly, the above alloy is not of course used in a molten salt atmosphere. U.S. Pat. No. 4,818,486 (Apr. 4, 1989), entitled “Low thermal expansion superalloy,” describes an Ni-based alloy including 8 wt. % of Cr, 25 wt. % of Mo, 0.003 wt. % of B, 1 wt. % of Fe, 0.5 wt. % of Mn and 0.4 wt. % of Si, which was prepared in order to develop a base material for a plasma spray type ceramic coating. However, the corrosion resistance of the above alloy in a molten salt atmosphere was not considered. U.S. Pat. No. 4,183,774 (Jan. 15, 1980), entitled “High-endurance superalloy for use in particular in the nuclear industry,” discloses an Fe, Ni or Co based alloy including 0.2 to 1.9 wt. % of C, 18 to 32 wt. % of Cr, 1.5 to 8 wt. % of W, 15 to 40 wt. % of Ni, 6 to 12 wt. % of Mo, 0 to 3 wt. % of Nb—Ta, 0.2 wt. % of Si, 0 to 3 wt. % of Mn, 0 to 3 wt. % of Zr, 0 to 3 wt. % of V, 0 to 0.9 wt. % of B and less than 0.3 wt. % of Co, however, corrosion resistance of the patented alloy in an electrolytic reduction molten salt atmosphere was not considered in view of use thereof. This alloy has a composition different from that of the present invention. Accordingly, a novel alloy with excellent corrosion resistance in a LiCl—Li2O molten salt system is still not yet reported.
A number of researches and studies for treatments of oxide spent nuclear fuels have been actively conducted in Korea and other advanced countries including the United States, Japan, and so forth. Especially, in order to treat the oxide spent nuclear fuel, investigations into metallization of the fuel in a LiCl—Li2O molten salt atmosphere via electrolytic reduction are underway. Such spent nuclear fuel after metallization can be directly processed into a metal nuclear fuel for a high speed furnace through a molten salt electrolytic refining process, therefore, may be considered as an effective technical strategy for treatment of an oxide spent nuclear fuel. However, the LiCl—Li2O molten salt which is an electrolytic reduction electrolyte has strong corrosive properties to conventional structural materials, therefore, makes it difficult to select an appropriate structural material for electrolytic reduction equipment with high reliability.
Recently developed corrosion resistant alloys commercially available in the art are in general alloys designed to attain favorable corrosion resistance against a high temperature oxidative gas and/or an oxidative aqueous solution. However, an improved alloy with corrosion resistance at 650□ in a LiCl—Li2O molten salt atmosphere which is a condition for electrolytic reduction of spent nuclear fuel is still not developed. From experimental results for the commercial alloys, it was determined that all commercial alloys have corrosion rate of more than a reference level of 0.5 mm/yr. Among such commercial alloys, Inconel 713 LC with the most excellent corrosion resistance exhibited a corrosion rate of at least 1.5 mm/yr measured under electrolytic reduction conditions, that is, in a LiCl-3 wt. % Li2O molten salt atmosphere. Therefore, it is difficult to use the above alloy in industrial applications.
Accordingly, there is still a need for development of a novel material with more reduced corrosion rate sufficient to use in hot cell working environments requiring high reliability.